1. Field of the Invention
The present invention relates to a communication device for transmitting digital audio and video data in an isochronous communication mode. More specifically, when digital audio and video data according to a standard definition (SD) is transmitted to another digital data device, the invention decides whether to transmit an empty packet or a DIF source packet for respective isochronous cycles, and controls transmission of the packets, whereby the invention is capable of transmitting digital audio and video data at a uniform data transfer rate in a predetermined margin.
2. Description of the Related Art
Development of computer technology in recent years introduced a digital multimedia era by linking computers with electronic home appliances. The computer technology allowing development of digital multimedia growth includes (a) improved technology of various digital multimedia equipment such as digital video cassette recorders (VCR) and digital camcorders and (b) interface technology which enables the multimedia equipment to be linked with each other. One of the most well-known interfaces is the IEEE 1394 interface. The IEEE 1394 interface is a standard bus interface definition conventionally used to network personal computers, portable computers and other digital systems which operate independently.
FIG. 1 is a block diagram of a multimedia system for data-communication with a digital camcorder using the IEEE 1394 interface. The multimedia system includes (1) a camera 11 for recording real world images and generating audio and/or video signals, (2) a signal processor 12 for processing the signals and producing audio and/or video information, (3) error correcting code means (FCC) 13 for appending error correcting code to the audio and/or video information, (4) a deck 14 for storing the information, (5) an interface 15 for interfacing the system 10 with another multimedia system 20 to provide mutual communication and (6) communicating means 17, which may be implemented by IEEE 1394 for performing digital communication between the multimedia systems 10 and 20. The data communication between the components of the multimedia system 10 is performed through inner bus 16.
When a frame is transmitted on the inner bus 16 during a bus interval, the inner bus 16 exhibits a high transfer rate at specific parts of the interval and stays idle during other parts to provide efficient audio and video data transmission within the device. The characteristics of the bus system are well known to those skilled in this art.
FIG. 2 shows a model of transmission of a stream of source packets from one digital device to another. A source packet may be split into 1, 2, 4 or 8 data blocks, and zero or more data blocks are contained in an IEEE 1394 isochronous packet. A receiver of the packet collects the data blocks in the isochronous packet and combines them to reconstruct the source packet for delivery to the application.
The source packets should be modified to comply with both the specification and the definitions for IEEE 1394 and DVC DIF(digital video cassette digital interface) to allow transmission from one digital device (e.g., a digital camcorder 10 shown in FIG. 1) to another digital device (e.g., a multimedia device 20 shown in FIG. 1). These IEEE 1394, SD specifications and definitions are commonly known by those skilled in this art so that only a few brief comments are sufficient before describing the operation of the present invention.
[Definition 1] Data transmission according to Standard Definition-Video Cassette Recorder Digital InterFace(SD-VCR DIF) should be applied with the following time conditions.
1 Start timing information of one frame should be transmitted to the receiver. The start timing information is carried in CIP header in the source packet of the frame, as time stamp information. The CIP header is placed at the beginning of the data field of an IEEE 1394 isochronous packet and contains information on the type of the real time data contained in the data field following the CIP header.
2 An n.sup.th data block of an M.sup.th video frame should be transmitted in a packet which meets the following conditions. (n=0 .sup.. . . K-1)
Packet_arrival_time_L&lt;T.sub.n PA1 T.sub.n -Transmission_delay_limit&lt;Packet_arrival_time_F PA1 T.sub.n is the nominal timing for data block n, EQU T.sub.n =T.sub.M -(T.sub.M+1 -T.sub.M)*n/K PA1 Packet_arrival_time_L: The cycle time when the last bit of the packet which includes data block n arrives in the receiver. PA1 Packet_arrival_time_F: The cycle time when the first bit of the packet which includes data block n arrives in the receiver. PA1 T.sub.M is the time stamp for video frame M. PA1 Transmission_delay_Limit=450 .mu.s PA1 K is the number of source packets in a video frame. For example, EQU K=250(525-60 system) EQU K=300(625-50 system)
where
The time conditions for frame transmission are generally described above. More detailed specifications are described in the Digital Interface and the CIP Header in the Blue Book of SD-VCR, published in December, 1995.
From the above definition, it is noted that the 250 or 300 source packets should be uniformly distributed in a fixed margin to allow data transmission between digital data devices.
[Definition 2] Isochronous transmission of IEEE 1394 operates by transmitting isochronous packets, responding to a cycle sync every 125 .mu.s as shown in FIG. 3. Detailed specifications on the IEEE 1394 are described in the standard published in 1995.
FIG. 4 shows the state of the inner bus 16 during transmission of one frame of data. As set forth above, the inner bus of the digital camcorder 10 usually has a high transfer rate at only specific parts in the frame.
The following observations are derived from the above definitions.
In the case of the 525-60 system (i.e., National Television System Committee: NTSC), the number of source packets is 250 per frame and the number of cycle sync per frame is approximately 266.9. As a result, in the 525-60 system, empty packets without data should be allocated to 16 or 17 isochronous cycles in order to transmit image information of a frame through the IEEE 1394 bus. Accordingly, the timing of the inner bus of the digital camcorder should be synchronized with the output timing through the IEEE 1394 bus. The empty packet insertion is to be determined to conform with [Definition 1, 2].
In order to synchronize the transfer rate of the inner bus of digital devices (e.g., a camcorder) and the transfer rate of IEEE 1394 bus under [Definition 1, 2], the steps of buffering transmission data using a memory, for example, and controlling the output timing of the transmission data from the memory are required.